Pigment mutants of Chlamydomonas with deformed plastids and increased levels of chloroplast nucleic acids.

Abstract

The chloroplast ultrastructure of two Chlamydomonas reinhardtii pigment variant mutants, U3N and U3A, is strikingly different from that of the wild type. The mutant chloroplast has greatly lowered levels of chlorophyll a and b, and lacks the usual ordered thylakoid membrane structure. The amount of chloroplast ribosomes is increased, but the pyrenoid and surrounding starch grains appear to be unaltered. Our biochemical analyses have shown that, while the properties of chloroplast DNA, ribosomal RNA, and ribosomes in these mutants appeared to be normal, their relative amounts per cell increased markedly when compared to the wild type. In U3N these increases were approximately 60% for chloroplast DNA and 80% for chloroplast ribosomes. However, the ratio of chloroplast rDNA genes to total chloroplast DNA remained unchanged as shown by DNA-rRNA hybrdization. We propose that (1) The enhanced level of chloroplast ribosomes in these mutants is a direct consequence of the elevated amount of chloroplast DNA. Both of these increases may, in turn, arise from defective mechanism for their control. (2) These mutants grow successfully in the absence of functional photosynthesis, provided an external carbon source is available to them, but functional plastid DNA, ribosomes and protein synthesis may still be a requirement for normal starch metabolism.